1. Field of the Invention
The present invention relates to microwave circuits. More specifically, the present invention relates to multiband microwave filter banks utilizing microelectromechanical switches.
2. Description of the Related Art
Multiband microwave filters are used in a variety of applications, such as communications, electronic warfare (EW), and instrumentation devices, to separate and identify signals in different frequency bands. Design requirements for these filters often include low cost, small size, light weight, low power dissipation, and high performance.
For certain applications, two of the most critical specifications are cost and size. A microwave filter bank is typically implemented in a miniature microwave multi-chip module (MCM). Presently, all microwave multi-layer technologies, i.e., Duroid, LTCC (low temperature co-fired ceramic), Alumina, etc., are in their infancies, requiring tight process controls to meet the demands of high performance buried circuits such as bandpass filters. Such a tight control on material and processing increases the unit production cost to be prohibitively high. The most common technology for filter miniaturization uses lanthanum aluminate (LaAlO3) substrates. These types of material are used exclusively in conjunction with low temperature superconducting (HTS) films. Such substrates are expensive, suffer from a high dislocation density, and have a relatively low dielectric constant, which results in a large, heavy circuit.
A common design for a multiband microwave filter connects a set of bandpass filters between two single pole multiple throw switches. These switches are typically implemented with semiconductor elements such as transistors or PIN diodes. At microwave frequencies, however, these devices suffer from several shortcomings. PIN diodes and transistors typically have an insertion loss (the loss across the switch when the switch is closed) greater than 1 dB, and an isolation value of less than 20 dB. This low level of isolation allows a signal to “bleed” across the switch even when the switch is open. In addition, PIN diodes and transistors have a limited frequency response and consume around 100–300 mW of DC power. These switches therefore increase the insertion loss and power dissipation of the overall multiband filter.
Hence, there is a need in the art for an improved multiband microwave filter bank that offers lower cost, smaller size, lighter weight, and improved performance over prior art filters.